Just thought I'd pop in here, cos I did UNIQ and Sutton Trust last year. Sutton Trust was at Imperial College for Engineering and UNIQ was for Engineering Sciences and I was at Wadham College (awesome college!). I'm sure you'll get help from the official ST or UNIQ TSR profiles, but I'm here to answer your questions too if you wish!
Both the Summer courses were extremely helpful for me either by giving me a greater insight into engineering/helping us through the application process/helping me decide which universities are right for me and so on. I made loads of friends on these courses with whom I still talk to now!
UNIQ was just a different experience altogether. We lived for a week as if we were Oxford undergrads. The mentors there were excellent - you could literally ask them anything, and the entire programme was awesome (at least for my course anyway).
For Sutton Trust I applied to Imperial because I knew it would be a university that would be first choice in my list and it didn't disappoint. For my course we were challenged thoroughly and it gave me a fair representation of what the university life would be like. Not to mention you get to be with students who are doing different subjects to you so you're not with the same dull people all the time who talk about numbers 24/7.
All in all, great experiences, and even if you don't think you'll get in you should still apply because they will certainly help you with your application, if not, at least give you a better understanding of what the university life has in store for you.
P.S. I ended up using half of the personal statement I wrote for UNIQ, so it gave me a head start!